Subject: question about CHAP timeout

From: "Matt Harris" <[email-masked]>
Date: Fri, 8 Sep 2000 12:42:56 +1200 (NZT)

I've got a situation (as below) where a peer is being slow to respond 
to an initial CHAP Challenge, which causes the authenticator to 
timeout and send a second Challenge, incrementing the ID as 
required by RFC 1994.  The peer then responds to the first 
challenge, and this response is discarded as the ID doesn't match 
the last challenge sent.

<---------------------- 
Challenge(ID1)
   
timeout

<----------------------
Challenge(ID2)

---------------------->
Response (ID1)  

Is there any reduction in security to accept this CHAP response 
and send the CHAP success message if the response is valid?  
Another solution would be to increase the timeout - does this have 
any security issues?

I see that Jame Carlson said there was no reduction in security for 
PAP, but is CHAP different?

TIA,

Matt

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Matt Harris,
R&D Software Development Engineer.

CentreCOM Systems Ltd.
An Allied Telesyn Group company - Simply connecting the World.

Unit 2, 242 Ferry Rd,            Phone: +64 3 377 8900.
PO Box 10-290,                   Facsmile: +64 3 377 8870.
Christchurch 8030,               Mobile: +64 21 701 762
New Zealand.                     	email: [email-masked]
                                 		http://www.alliedtelesyn.co.nz/
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

---

From: James Carlson <[email-masked]>
Date: Fri, 8 Sep 2000 06:22:28 -0400 (EDT)

> challenge, and this response is discarded as the ID doesn't match 

The usual solution to this is to use an increasing restart value
(timeout).  For example, 3 seconds the first time, 6 the next, and so
on.

> Another solution would be to increase the timeout - does this have 

If you then spoof out Response with ID 2 (send a copy of the last
Success or Failure message with ID 2 but without checking the Response
value), then I don't see a problem.  It's as though you never sent
Challenge with ID 2.

I'm not a security expert, though, and if the security of the system
is important, you may want to consult with one.

> PAP, but is CHAP different?

Unless I'm mistaken, the statement on PAP had to do with accepting
malformed Authenticate-Ack messages.  Since those messages merely
imply the *peer's* acceptance of the Authenticate-Request, it's not a
problem if they're slightly malformed (such as missing the required
message length octet).  Accepting those (and malformed CHAP Success or
Failure messages) does no obvious harm.

-- 
James Carlson                                  <[email-masked]>
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp